1. Field of the Invention
The present invention is in the field of logging instruments for evaluation of cement bonding to pipe casings in downhole formations. More particularly, the present invention is directed to improved acoustic logging instruments for evaluation of cement bonding to pipe casings and also in open holes in downhole formations.
2. Brief Description of the Prior Art
A pipe casing in a downhole formation utilized for the production of oil, gas steam or other minerals is typically surrounded by a layer of cement that ideally should be tightly bonded to the metal casing. The integrity of the bond between the metal casing is of such importance that instruments or "tools" have been developed in the prior art to measure the integrity or quality of the casing-metal-to-cement bond and to create a "log" of the corresponding data along the length of the pipe casing.
The best known instruments utilized for the above-noted purpose operate on acoustic principles. More particularly, the acoustic instrument utilized for this purpose includes a sound emitting transducer and a plurality (usually two) sound sensors positioned at predetermined distances (usually at 3 and 5 feet) above the transducer. The instrument is lowered into the borehole on a wireline, centered within the casing by means that are usual for centering downhole instruments in a pipe and the transducer is activated by electrical energy supplied to it through the wireline. Sound waves (acoustic energy) generated by the transducer travel through several paths to the two sensors located in the instrument above the transducers. One eminent path of the sounds is through the liquid (water or drilling mud) that usually fills the pipe casing at this stage of the downhole operation or exploration, another paths is through the metal casing, still another through the formation, and yet another through the body of the instrument itself. The basic principle behind using acoustic energy to collect data on the integrity of cement bonding to the casing is that cement bonded tightly to the metal casing significantly attenuates the sound energy that is conducted through the pipe, much the same as a steel tube held tightly in a vise "rings" significantly less when struck by a hammer than a free standing steel tube. Thus, it is important for the instrumentation that receives data from the sensors to identify the acoustic energy that reaches the sensor(s) through the casing, and distinguish it from acoustic energy that has traveled to the sensor(s) through other routes.
Generally speaking sound waves (acoustic energy) travel through aqueous fluid at the speed of approximately 180 to 220 .mu.sec/foot, through steel at approximately 57 .mu.sec/foot and in the formation at the speed of approximately 45 to 200 .mu.sec/foot. Based on these different speeds an instrument that receives input from the sensors that measure the timing of the sound waves' arrival as well as their intensity (amplitude) can usually differentiate on the basis of the timing of their arrivals (and other factors) among the sound waves that have traveled from the transducer to the sensor(s) through the liquid inside the casing, the steel pipe and the formation. However, it was found in practice that the sound waves traveling through the metal body of the instrument itself are difficult to distinguish from the sound waves (acoustic energy) that reaches the sensor(s) through the metal casing. Moreover the acoustic energy transmitted through the body of the instrument carries no useful information regarding the formation nor about the integrity of the cement bonding to the casing.
The prior art has coped with the just-described problem in various ways. One method of solution utilized in the prior art is to place a lead-filled pipe section as part of the body of the instrument, that is separating the sound emitting transducer of the instrument from the sensors by a lead-filled pipe section that acts as an "isolator bar". However it was found that such an isolator bar functions to block (or significantly reduce) the transmission of sound energy through it only when the lead filling is tightly bound to the interior walls of the pipe. This bonding deteriorates with repeated exposure of the instrument to the high pressures downhole coupled with repeated returns to atmospheric pressure on the surface. Another method of solution in accordance with the state-of-the-art is to externally coat a steel pipe or bar with lead, and use that as an isolator bar between the transducer and sensors. Still another method is to provide an isolator bar that comprises a highly slotted steel body so that the sound waves traveling through it must travel through multiple and extended paths whereby they arrive later than they would through an ordinary pipe section, and tend to cancel each other due to interference.
Although each of the solutions described above has been found workable, neither of them is ideal. One significant problem associated with the use of acoustic instruments of the type described above concerns the utilization of space. As is well known, in the highly confined environment of downhole instrumentation space available for packaging of components must be utilized well. In other words, electronic and other equipment must be packed as tightly as possible within the relatively narrow, usually cylindrical interior of the instruments. Moreover, the one method of providing more space by extending the overall length of the instrument is practiced, but is not preferred in the art. The use of a lead-filled tube as an isolator bar as practiced in the prior art does not permit the placing of components along the length of the bar, and therefore undesirably extends the overall length of the instrument. This is particularly disadvantageous because it is customary in the art to combine the use of the acoustic logging tool with a .gamma.-ray tool that measures .gamma.-ray radiation of the formation along the length of the pipe casing. In state-of-the-art instruments the .gamma.-ray tools are usually coupled to, that is are physically attached to the acoustic logging tool, below the acoustic tool. Usual length of state-of-the-art acoustic logging tools is approximately 9 to 10 feet, and the usual length of state-of-the-art .gamma.-ray tools is approximately 4 feet, thus adding up to an overall length of approximately 13 to 14 feet for the acoustic logging tool and .gamma.-ray tool combination, not including the length of the usually appended centralizer and casing collar locator. The present invention allows significantly better utilization of space and provides a significantly shorter combination of the acoustic logging and .gamma.-ray tools.